Writing instruments provided with a lateral button for causing the writing tip to advance, as described for example in document U.S. Pat. No. 3,883,253, have the advantage for the user of keeping the instrument in the writing position in his hand when it is necessary to cause the writing tip to move forward or to protrude. With this type of writing instrument, it is at most necessary for the user to move his index finger along the body to actuate the lateral button. This is particularly advantageous with propelling pencils which require regular actuation of the lead advancing mechanism as the lead is used up.
However, the lateral buttons of the prior art have the disadvantage of being able to cause a relatively slight longitudinal movement of the advancing mechanism. Specifically, the active stroke of the button's cam against the ramp of the advancing mechanism is limited by the internal diameter of the tubular body and, consequently, it is not possible to obtain a considerable stroke without excessively enlarging the diameter of the body and while retaining an appropriate incline of the ramp, so that the pressure to be exerted on the button remains acceptable. In addition, because of the presence of a tube in the center of the tubular body, such as for example a lead storage compartment, the lateral button is prominent on the outside of the body, which may make it awkward to hold the writing instrument and make it more difficult to actuate the lateral button without changing the hand gripping of the writing instrument.